Hexaarylbisimidazole (hereinafter also called ‘HABI’)(Non-Patent Document 1), diarylethene (Patent Document 1), spirooxazine (Patent Document 2), etc. are known as photochromic compounds which possess characteristic of photochromism. Since these compounds possess the characteristic of color reversal when being irradiated by light, the research of the compounds has been actively carrying out aiming to an application as taking the compounds as light control materials (Patent Document 3) or photorecording materials (Patent Documents 4 and 5).
Hexaarylbisimidazole (HABI) generates a triarylimidazoleyl radical (hereinafter also called a ‘TAIR’), which is a highly reactive radical material when irradiated by UV and is therefore conventionally widely used as a photopolymerization initiator (Patent Documents 6 to 8).
The photochromic compounds is well-known as P-type photochromic compounds and T-type photochromic compounds. After being isomerized when irradiated by light, the P-type photochromic compounds can reversibly recover to the original structure by irradiation of light having wavelength different from the previous irradiated light. The T-type photochromic compounds, after being isomerized when irradiated by light, can reversibly recover to the original structure by applying a thermal reacting process for a time period from several minutes to several hours.
However, it is with a drawback that it is necessarily to take a time period from at least several minutes to several hours for performing discoloring reaction since these known conventional photochromic compounds need to transform between isomers of different structures. Furthermore, the conventional HABI has another problem that since a carbon-nitrogen bond is cleaved to generate two triarylimidazoleyl radicals and then the two triarylimidazoleyl radicals will diffuse in medium in such a manner that it takes time for the recombination of radicals, therefore slowing the discoloring reaction rate and lowering the long-term stability such as repetition durability.
In one attempt to solve these problems, it has disclosed a synthesis procedure that two triarylimidazoleyl radical molecules are introduced into the 1- and 8-positions of naphthalene (1,8-NDPI-TPI-naphthalene) (Non-Patent Document 2). It conjugates the solely two naphthalene skeletons and an imidazoleyl radical to form a resonant structure so as to stabilize the chromogen, thereby suppressing the diffusion of radicals. However, not only the above attempt cannot fully satisfy the requirement for discoloring speed but also it has a problem that the chromogen, which is a stabilized radical material, will extract hydrogen from a surrounding medium to thus cause degradation.
Furthermore, there is another method to solve this problem. It has disclosed a synthesis procedure that a triarylimidazoleyl radical is introduced into the paracyclophane introduces to synthesis pseudogem-bisDPI [2.2]paracyclophane (Patent Document 9). The two triarylimidazoleyl radical (TAIR) is bound by using a bridging group which is not conjugated with TAIR to thus it enhances a higher thermal stability, an excellently long-term stability, a high speed of discoloring and higher density coloring. However it can not fully satisfy the requirement of high speed for color switching and of higher density for coloring.
Furthermore, there are a plurality of manufacturing processes required and the material of [2.2]paracyclophane is expensive such that the manufacturing cost thereof is high to thus cause a problem in view of the industrial utility.
Moreover, it has disclosed a synthesis procedure that synthesizes two triarylimidazoleyl radical (TAIR) by use of a conjugated bridging group to control coloring density and discoloring speed after a visible light is emitted thereon (Patent Document 10). In spite that it applies a light having a wide range of visible spectrum to thus achieve a high coloring density, it has the same problem as the pseudogem-bisDPI [2.2]paraclophane that it is incapable of fully satisfying the requirement of high speed for color switching and of higher density for coloring.